It is already known to provide reliable storage and retrieval of large volumes of digital data, such as computer data, in particular by means of the DDS (Digital Data Storage) format defined in ISO/IEC Standard 10777:1991 E and implemented in helical scan tape drives made by several companies.
In the DDS system an elongate recording medium comprising tape coated with a magnetic medium is moved by a motor-driven capstan along a path wrapped partially around a transducer comprising a rotating drum carrying one or more electromagnetic heads. The plane of rotation of the drum is disposed at an angle to the plane of movement of the tape, so that each head traverses the tape along successive tracks extending across the width of the tape at an angle to its centreline.
In helical scan data storage devices, it is sometimes necessary or advantageous to employ adaptive filtering in the processing of the signal(s) read from the read head(s). An adaptive filter automatically adjusts its response according to certain properties of the incoming signal. Adaptive filters are well-known and an example of an adaptive filter used in a backup storage device can be seen in Applicants' U.S. Pat. No. 5150379. A typical characteristic of such a filter is a tendency to adopt an undesirable response when presented with a signal which has a spectral content which differs from the normal data signal. In helical scan recording, it is common for such signals to be recorded in margin or preamble parts of the track format.
Typically the precise location of each track is determined during recording of data on the tape by the position relative to the tape of write heads on the drum. To ensure optimum retrieval of data from the tape, it is desirable for read heads on the drum to follow paths across the tape which are at essentially identical positions to those followed by the write heads during data recording. This in turn requires control during data retrieval of relative motion between the tape and the drum, by controlling rotation of the drum and/or controlling movement of the tape by the capstan.
A system for providing such control is described in Applicants' co-pending published European Application No. 671735 and is incorporated herein by reference. In that system, the rotary head drum generates a position signal at one or more predetermined angular positions which is used to control the motion of the media.
This drum position signal is also used as a reference for the generation of a timing signal to enable and inhibit adaption of the filter characteristics according to the position of the read head in the track.
However, it is typical for the height of the recorded tracks relative to the reference edge of the medium to vary according to environmental factors, and across a population of data storage devices. It is therefore necessary to incorporate considerable timing margin in the signal which enables and inhibits filter adaption, so as to guarantee that adaption does not take place at a time when the signal from the read head has undesirable spectral characteristics. As a result, adaption of the filter characteristics is normally enabled relatively late, to the detriment of error performance achieved on the early part of the data.
The present invention aims to overcome this limitation.